The present invention relates to a method for controlling a hybrid drive which includes an internal combustion engine and at least one electric motor which can be driven via an electrical energy source, and drives a vehicle and in which the electric motor is switched over to generator mode when a preset requirement to generate electricity is present.
German Patent Document DE-OS 25 01 386 discloses a method of the above-described type, in which the electric motor which is provided to drive the vehicle operates when required as a generator driven by the internal combustion engine in order to produce current, for example for charging up the battery feeding the electric motor.
An object of the present invention is to provide a method of the initially described type in which the fuel consumption of the internal combustion engine and its emission of noxious substances are further reduced.
This and other objects are achieved by the present invention which provides a method for controlling a hybrid drive that drives a vehicle, the hybrid drive including an internal combustion engine and at least one electric motor which can be driven via an electrical energy source. The method comprises the steps of switching the electric motor over to a generator mode when a preset requirement to generate electric energy is present, and operating the internal combustion engine substantially at a torque corresponding to ideal operation of the internal combustion engine with respect to at least one of efficiency and exhaust gas behavior when the internal combustion engine is in operating ranges in which an amount of torque to be output by the internal combustion engine for overcoming instantaneous tractive resistances lies below a torque corresponding to the ideal operation of the internal combustion engine. A differential torque between the torque corresponding to the ideal operation and the torque to be output by the internal combustion engine for overcoming the instantaneous tractive resistances is used to drive the electric motor operated as a generator.
Using the method according to the invention, it is achieved that whenever the vehicle is being driven via the internal combustion engine and there is a need to generate electric energy, for example to charge up a battery which is supplying energy for the electric motor or for other electrical loads, the internal combustion engine can be operated over wide ranges always in an operating range which is most favorable with respect to the specific fuel consumption and/or with respect to the emission of noxious substances. The taking up of the torque of the electric motor running in generator mode is controlled in this process in such a way that the precise difference between the torque output by the internal combustion engine in these load ranges and the torque required to overcome the instantaneous tractive resistances, provided this difference is greater than zero, is taken up by the generator to generate electric energy. Both the storage of the electric energy required to operate the electric motor and the drive of the vehicle via the internal combustion engine can thus be achieved in these operating ranges with optimum efficiency and/or with minimum emission of noxious substances.
In certain embodiments of the present invention, the control of the hybrid drive takes place according to a characteristic diagram. This has the advantage that the most varied parameters can be taken into account for the control of the internal combustion engine and of the generator so that changes in individual operating parameters and ambient parameters can be reacted to as quickly as possible.
In certain embodiments of the invention, a battery is coupled to the electric motor and is an energy source to drive the electric motor. The take-up of the torque of the electric motor operating in the generator mode is controlled to a set value which is greater than the torque corresponding to ideal operation, whenever the battery is discharged to a severe degree. With these embodiments, it remains ensured that in the event of the generation of a high electric energy being required briefly, for example when the battery is discharged to a severe extent and a plurality of electrical loads are switched on at the same time, this required electric energy can be provided as quickly as possible.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.